1. Field of the Invention
The invention relates to a method and apparatus of controlling energy flow between one or more power generating sources and multiple energy storage devices. Specifically, this invention manages energy transfer between at least two dissimilar energy storage devices utilized in an energy storage system for efficient transfer of energy based on both short term and long term energy fluctuations.
2. Discussion of the Related Art
In recent years, increased demands for energy and increased concerns about supplies of fossil fuels and their corresponding pollution have led to an increased interest in renewable energy sources. Two of the most common and best developed renewable energy sources are photovoltaic energy and wind energy. Other renewable energy sources may include fuel cells, hydroelectric energy, tidal energy, and biofuel or biomass generators. However, using renewable energy sources to generate electrical energy presents certain challenges.
One of the biggest challenges for connecting renewable energy sources to existing AC grids, whether the utility grid or an off-grid system, is that renewable energy sources often provide a variable supply of energy. The supply may vary, for example, according to the amount of wind, cloud cover, or time of day. Further, different energy sources provide different types of electrical energy. A wind turbine, for example, is better suited to provide Alternating Current (AC) energy while a photovoltaic cell is better suited to provide Direct Current (DC) energy. As a result, combining multiple sources of renewable energy with other generating systems, such as the utility grid, independent micro turbines and generators, or fuel cells into a single system with an AC and/or a DC output requires integration of each of these different energy sources.
The variable nature of the energy supplied by some renewable sources may also make it desirable to integrate an energy storage device in the power system. The energy storage device may be charged during periods of peak production by the renewable source or, alternately, by the utility grid or other generating source. The energy storage device may then deliver the stored energy to supplement the renewable source when the renewable source is generating less energy than is required by the loads in a system.
Most electrical energy storage devices are batteries which convert electrical energy to potential energy via a first chemical reaction when charging the battery and perform a second chemical reaction to convert the potential energy back to electrical energy when drawing power from the battery. The two chemical reactions are intended to be complementary and reversible such that energy may repeatedly be stored and drawn from the battery. Further, different types of batteries utilize different chemicals in their respective reactions. Some chemical reactions are better suited for rapid charging and discharging while other chemical reactions are better suited for slow charging and discharging.
A power converter is typically utilized to regulate the energy flow to and from the energy storage device. In many applications, a single energy storage device may be utilized and a single power converter is provided to regulate the energy flow to that storage device. In other applications, a higher amount of energy storage may be desired and multiple energy storage devices may be needed. It is known that multiple energy storage devices may be connected in banks to a single power converter or individually to multiple power converters. Historically, however, when a higher level of energy storage is desired, a single type of battery and/or power converter is selected such that the devices may charge and discharge in a uniform manner. If batteries having different chemical compositions are selected, batteries of one composition may charge/discharge at a faster rate than batteries of another composition. As a result, batteries of one composition may be prone to failure due to excessive cycling. Batteries of the other composition may be prone to premature failure due to partial charge/discharge cycles.
Thus, it would be desirable to provide an energy storage system that utilizes batteries of multiple compositions and provides improved load sharing between the different types of batteries.